An image forming apparatus such as a laser printer includes a photosensitive drum and a transfer roller opposed thereto, for example. An electrostatic latent image corresponding to an image to be formed on a sheet is formed on the surface of the photosensitive drum. A toner is fed to the electrostatic latent image, so that a toner image is carried on the surface of the photosensitive drum. When the photosensitive drum is rotated so as to oppose the toner image to the sheet transported between the photosensitive drum and the transfer roller on a position facing the transfer roller, the toner image is transferred from the surface of the photosensitive drum to the sheet due to the action of a transferring bias supplied to the transfer roller. Thereafter the sheet is heated and pressurized, so that the toner image is fixed to the sheet, thereby forming the image on the sheet.
In relation to such an image forming apparatus, there has been provided an apparatus having a so-called automatic duplex mode for forming an image on a first face of a sheet and thereafter inverting and transporting the sheet for forming another image on a second face of the sheet opposite to the first face.
In this automatic duplex mode, the image is formed on the second face of the sheet immediately after the formation of the image on the first face, whereby the sheet exhibits different electric resistances in the image formation on the first face and that on the second face. In other words, the sheet having the image formed on the first sheet is dried due to heating for fixing the toner image thereto, so that it exhibits a higher electric resistance than that before the image formation. In order to transfer the toner image onto the second face of the sheet in an excellent state, therefore, a transferring bias for the image formation on the second face of the sheet must be higher than that for the image formation on the first face. There have been proposed some methods of controlling transferring biases in such an automatic duplex mode.
Conventional image forming apparatuses include that having the so-called manual duplex mode where an image is formed on a first face of a sheet and the sheet is ejected onto a sheet ejection tray and then the user sets the ejected sheet on a sheet feeding tray and starts image formation on a second face of the sheet.
In this manual duplex mode, however, no control is performed for attaining excellent image formation on the second face of the sheet, dissimilarly to the aforementioned automatic duplex mode controlling the transferring biases. Further, there has been no proposal related to such control at present.